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Colour shifting demantoids from Afghanistan

Nick_G

Rough_Rock
Joined
Sep 7, 2018
Messages
76
OK, this is about a mineral specimen rather than a cut stone, but I was wondering if any of the experts here know anything more about this deposit. I recently acquired a big specimen of green garnet, which I believe is from Ghazni Province in Afghanistan. It's basically a big hunk of serpentinite-type rock, the top surface of which is maybe 2/3rds covered in glittering green andradite var. demantoid crystals. Pic here, although in daylight the green is much livelier in person:

Demantoids in matrix Afghanistan.JPG

It's interesting how the crystals are sitting on a creamy white fibrous mineral, rather than directly on the rock surface. The crystals are a beautiful mint green in daylight, with some more yellowish green crystals mixed in. Under a tungsten bulb, the mint green colour is muted and washed out, similar to how it appears in the photo. However, under an energy-saving light bulb the crystals change to a rich lime green colour. It's quite an obvious colour change. I'm guessing there must be a chromophore such as Cr or V interacting with the essential iron in there that's responsible for this.

For reference, the specimen is approx 16 cm by 14.5 cm by about 5.5 cm deep, although it is 7 cm deep at one end, and the weight is 1882.5 grams. It's a big piece and my largest specimen to date.

I'd be interested to read any gemmological discussions of this deposit or if anyone here has studied any specimens and can tell me more.

Thanks and regards,
Nick
 

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T L

Super_Ideal_Rock
Premium
Joined
Sep 20, 2008
Messages
22,858
Namibian stones shift as well from a more yellowish green to a bluish green.
 

Nick_G

Rough_Rock
Joined
Sep 7, 2018
Messages
76
Thanks T L. That's interesting to know. I found an article on the demantoids from Madagascar, but which also referred to the Namibian specimens:

https://www.gia.edu/doc/Spring-2011-Gems-Gemology-Demantoid-Topazolite-Antetezambato-Madagascar.pdf

An interesting quote from that article:

The color-shift behavior shown by some samples is similar to that seen in demantoid from Quebec,Canada (Amabili et al., 2009). The rare-earth elements cerium and neodymium, though detected only in trace amounts in our specimens, could be involved in the color shift, as documented in color-change glass (Quinn and Muhlmeister, 2005) and in minerals such as apatite.
I wonder if something similar is responsible for this in my specimen?
 

Nick_G

Rough_Rock
Joined
Sep 7, 2018
Messages
76
Anyone else ever come across this material? Any more info?

Thanks.
 

T L

Super_Ideal_Rock
Premium
Joined
Sep 20, 2008
Messages
22,858
It’s pretty common for demantoids to color shift. They can be moody stones. I’m not sure about top quality colored material, but color shifting is pretty common in many gem species. Demantoid is no exception.

I have some, but they’re locked away. I can take pics eventually to display this.
 

Arkteia

Ideal_Rock
Premium
Joined
Nov 3, 2009
Messages
7,380
I have one Russian demantoid that is certified unheated, and it shifts colors from one beautiful (neon green) to another beautiful (different shade of green).

Demantoids in general take so much abuse in the form of heating, and possibly some other treatments, that they all must have PTSD.

Namibian demantoids have oily color and no “tails”, but are very dispersive. They do shift colors, of course, all garnets do. I think setting them with orange spessartines pulls out green in demantoids, and this is how I’d set them.
 

Arkteia

Ideal_Rock
Premium
Joined
Nov 3, 2009
Messages
7,380
So it was sold as Russian but I think it looks more Namibian, although Russian could be of such a different color, too. Not my top one, that one is hidden ))...but dispersion is out of another world, and if I had the second similar spessartite, I’d make a 3-stone ring of this 3-carater in a whisper.
 

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